1. Field of the Invention
The present invention relates to a key sheet for pushbutton key switches for use in operating portions of various electronic apparatuses such as a mobile phone, a PDA, and an AV apparatus, in particular, to a key sheet in which pushbutton key tops and a base sheet are firmly bonded together by means of adhesive.
2. Description of the Related Art
As a pushbutton switch key sheet for a portable communications apparatus such as a mobile phone, there is widely used a so-called illumination type key sheet which is composed of a translucent key top and a translucent base sheet and which effects illumination with light from within the apparatus. In this illumination type key sheet, the translucent key top and the translucent base sheet are bonded together by using translucent adhesive.
The adhesive is applied to the bonding surface of the key top or that of the base sheet by a dispenser, screen printing, etc. If the amount of adhesive applied is rather small, an unsightly adhesive mark 2 becomes visible on the back side of the key top 1 as shown in FIG. 8. In view of this, when performing adhesive application by the dispenser, a relatively large amount of adhesive is used. This may result in some adhesive being squeezed out of the bonding surface of the key top and the base sheet; the adhesive thus squeezed out may adhere to the periphery of the bonding surface of the base sheet to be cured thereon. Then, the base sheet will be deformed in an unbalanced fashion at the time of depressing operation, making it rather difficult to perform the depressing operation. Thus, as a technique for preventing the adhesive from being squeezed out, JP 2003-297181 A discloses a key unit structure in which a bank-like protrusion is provided on the bonding surface of the key top or that of the base sheet. Further, JP 2005-50709 A discloses a key sheet in which an adhesive accumulation groove is formed in the bonding surface of the key top or that of the base sheet. In the case of screen printing, the adhesive is applied very thinly and two-dimensionally to the entire bonding surface of the key top or of the base sheet, making the adhesive mark hard to be visible while preventing the adhesive from being squeezed out.
However, in the key unit structure disclosed in JP 2003-297181 A, the key unit is rather thick due to the provision of the protrusion on the bonding surface of the key top and of the base sheet. In the key sheet disclosed in JP 2005-50709 A, it is necessary for the key top or the base sheet to be thick enough to allow formation of a groove. Thus, in both of the conventional techniques, it is rather difficult to achieve a reduction in thickness. Further, as shown in FIG. 9, in the case of adhesive application by screen printing, the outer edge of an adhesive layer 3 is raised after the application as shown in FIG. 9. Thus, when bonding the key top 1 and the base sheet 4 to each other, bubbles are easily allowed to be mixed into the interface of the adhesive layer 3 after the application. As a result, as shown in FIG. 10, bubble marks 5 are visible through the key top 1, causing additional inconvenience.